Optical Illusion Explained in Monkey Brain Study

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Look closely at the FedEx logo and you'll notice the space
between the "E" and the "x" creates the outline of an arrow. Now,
a new study reveals the part of the brain that creates such
invisible shapes.

The FedEx arrow is just one example of a common
optical illusion, whereby the brain "sees" shapes and
surfaces within a fragmented background, although they don't
exist. Scientists studied the effect in monkeys, finding a group
of neurons in part of the visual cortex that fire when the
animals viewed an illusion pattern.

"Basically, the brain is acting like a detective," study leader
Alexander Maier, a psychologist at Vanderbilt University in
Nashville, Tenn., said in a statement. "It is responding to cues
in the environment and making its best guesses about how they fit
together. In the case of these illusions, however, it comes to an
incorrect conclusion."

The visual cortex, a part of the brain at the back of the head,
processes visual information in mammals. Scientists often divide
the visual cortex into five regions labeled V1 through V5.

Visual signals from the eyes go to the primary visual cortex, V1,
which detects their orientation, color and spatial arrangement.
The brain splits that information into two streams, known as the
dorsal and ventral streams. Both pathways go to V2, which makes
some connections to V3.

The dorsal stream, sometimes called the "where" pathway, connects
to V5, and is
involved in detecting motion and locations of objects,
as well as helping with hand-eye coordination.

The ventral stream, or the "what" pathway, goes to V4, and is
involved in form and
object recognition. Maier and his colleagues hypothesized
that V4 might be involved in producing illusory contours.

In the study, the scientists trained monkeys to stare at a screen
with an image of a Kanizsa square (a variant of the Kanizsa
triangle) — four "Pac-Man" shapes with their mouths arranged to
form the corners of a square. The square doesn't actually exist,
but the brain creates one by mentally connecting the dots.

When the monkeys were looking at the Kanizsa square, neurons in
V4 of their brains that were involved in representing the middle
of the square started firing. But when the monkeys saw the same
Pac-Man shapes facing outward, so they no longer framed a square,
those same neurons turned off.

The results, detailed today (Sept. 30) in the journal Proceedings
of the National Academy of Sciences, suggest V4 is the part of
the primate brain that represents illusory shapes.